Abstract: A membrane-based heat and mass exchanger (MHME) using hydrophilic membranes to recover the heat and mass of the exhaust air was analysed.Using the second law of thermodynamics, the process of the heat and mass transfer between the fresh and the exhaust in the MHME was studied.Under the typical summer operating condition, thermal entropy increase, mechanical entropy increase, latent entropy and total entropy increase were analysed. The entropy generation per quantity of heat exchanged (EGPH), which. reflects the process’s thermodynamic irreversibility was analysed. It showed that the spacing between the membranes influenced the mechanical entropy increase most. The results also indicated that the mass velocity of the gas, the spacing between two membranes and the length of the membranes would influence the EGPH. There existed an optimal spacing where the EGPH was the least.